Exopolysaccharide Produced by Lactobacillus Plantarum Induces Maturation of Dendritic Cells in BALB/c Mice

Lactobacillus plantarum (L. plantarum) exopolysaccharide (EPS) is an important bioactive component in fermented functional foods. However, there is a lack of data concerning the effects of L. plantarum EPS on maturation of mouse dendritic cells (DCs). In this study, we purified L. plantarum EPS and examined its effects on cytokines production by dendritic cells in serum and intestinal fluid of BALB/c mice, then investigated its effects on phenotypic and functional maturation of mouse bone marrow-derived dendritic cells (BMDCs). Cytokines (nitric oxide, IL-12p70, IL-10 and RANTES) in serum and intestinal fluid were analyzed by enzyme linked immunosorbent assay (ELISA) after the mice received EPS for 2, 5 and 7 days, respectively. DCs derived from bone marrow of BALB/c mouse were treated with EPS, then the phenotypic maturation of BMDCs was analyzed using flow cytometer and the functional maturation of BMDCs was analyzed by ELISA, and, lastly, mixed lymphocyte proliferation was performed. We found the molecular weight of purified EPS was approximately 2.4×106 Da and it was composed of ribose, rhamnose, arabinose, xylose, mannose, glucose and galactose in a molar ratio of 2:1:1:10:4:205:215. We observed that L. plantarum EPS enriched production of nitric oxide, IL-12p70 and RANTES, and decreased the secretion of IL-10 in the serum or intestinal fluid as well as in the supernatant of DCs treated with the EPS. The EPS also up-regulated the expression of MHC II and CD86 on DCs surface and promoted T cells to proliferate in vitro. Our data provide direct evidence to suggest that L. plantarum EPS can effectively induce maturation of DCs in mice.


Introduction
Lactobacillus plantarum (L. plantarum), a Gram-positive bacteria commonly found in nature, has industrial importance as a key component of fermenters used in probiotic fermented milk

Ethics statement
Our study was carried out in accordance with the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health (2011) and Guidelines for the Euthanasia of Animals of American Veterinary Medical Association (2013). The protocol was approved by the Committee on the Ethics of Animal Experiments of Northeast Agricultural University (Permit Number: 20130518-03). These animals were anesthetized by injection of sodium pentobarbital (50 mg/kg) to obtain blood from their hearts, and then sacrificed by cervical dislocation to remove their small intestines for the intestinal fluid. All efforts aimed to minimize suffering of the mice. No mouse became ill or died during the experiment.

Chemicals
In this study, lipopolysaccharide (LPS), as positive control, was purchased from Sigma-Aldrich and RPMI 1640 complete medium (RPMI 1640) were sourced from Thermo Fisher Scientific Inc. (Gibco, US). The NO assay kits were products of Hangzhou Sijiqing Co. (Hangzhou, China). The fluorescent monoclonal antibodies (PE-CD11c, FITC-MHCII and FITC-CD86) were purchased from eBioscience (San Diego, CA) and the ELISA assay kits for IL-12p70, IL-10 and RANTES were also products of eBioscience. Other reagents were mainly from either Sigma-Aldrich or Invitrogen.
Bacterial strains and culture conditions L. plantarum was purchased from the Institute of Microbiology, Chinese Academy of Sciences, China. It was isolated from fermented foods and was identified by 16S rDNA sequence alignment. The medium used for fermentation by L. plantarum producing EPS consisted of 12% skim milk powder, 1% peptone (Oxoid), 1.5% glucose (Oxoid) and 0.1% K 2 HPO 4 . All of these ingredients were mixed in water and heat treated at 121°C for 15 min. Fermentation was carried out at 37°C for 20 hours with a 3% inoculum.

Separation and purification of the EPS
After the incubation of L. plantarum for 20 hours, trichloroacetic acid was added to the cultures to a final concentration of 4% (w/v), and the cultures were stirred at 4°C for 12 h, then centrifuged at 10000 g for 20 min at 4°C. Finally, the supernatant was collected and concentrated to 25% of the original volume. EPS was precipitated by gradually adding an equal volume of cold ethanol and was collected by centrifugation, then diverted into dialysis bags (MW 7000~14000) which were immersed in deionized water. The water was refreshed once every 8 h for 3 d. Finally, the aqueous EPS solutions were freeze-dried in a DURA-DRY freeze-dryer and the EPS lyophilized powder was collected for later study. The content of total sugar in the powder was determined by a colorimetric method as previously described [20] and the absorbance was measured at 490 nm in a spectrophotometer. The concentration of protein was measured by the Bradford method using a protein assay kit following the manufacturer's instructions. The content of endotoxin in the powder was determined using a chromogenic Limulus amedocyte lysate kit (Associates of Cape Cod, USA).

Monosaccharide and molecular weight (Mw) determination for the EPS
The monosaccharides determination of L. plantarum EPS was carried out by gas chromatography (GC). The Mw of the polysaccharide was determined by size exclusion chromatography (SEC). GC and SEC analysis were performed referring to protocols reported in previous work [21]. Five dextrans standards (Mw 344. 8, 606.2, 1185, 1907 and 2800kDa) were used to establish a linear regression for Mw determination.

Mice and feeding
BALB/c female mice (6~8 weeks old, 18~22 g) were purchased from Charles River (Beijing, China). These animals were randomly divided into phosphate buffered saline (PBS) group, EPS group and LPS group and they were housed in plastic cages kept in a specific-pathogenfree (SPF) atmosphere with temperature 23± 2°C, humidity 55 ± 2% and a 12 h light/dark cycle. Mice management was in accordance with the guide for the care and use of laboratory animals of the National Institutes of Health.
L. plantarum EPS or LPS was dissolved in PBS solution. Tested mice received 100 mg/kg body weight/day of EPS by gavage for 2, 5 or 7 days, while positive control mice received 1 mg/ kg body weight/day of LPS and negative control mice received PBS instead. All mice received a basic balanced diet and water ad libitum. After 2, 5 and 7 days of administration, these animals were anesthetized respectively by injection of sodium pentobarbital (50 mg/kg) to obtain blood from their hearts, then they were sacrificed by cervical dislocation to remove their small intestines for the intestinal fluid.

Cytokine determination in serum and intestinal fluid
The blood was kept at 37°C for 1 h, and then centrifuged at 6000 g for 10 min at 4°C to obtain the blood serum. Mixture in the small intestine of the mice was flushed with PBS and it was centrifuged at 10,000 g for 10 min at 4°C to get supernatant. The supernatant and the serum were kept frozen at -80°C until use. NO activity was determined using NO assay kits (Nanjing Jiancheng Bioengineering Institute, Nanjing, China), and IL-12p70, IL-10 and RANTES were quantified by commercial mouse IL-12p70, IL-10 and RANTES enzyme-linked immunosorbent assay (ELISA) kits following the manufacturer's instructions.

Induction of BMDC
BALB/c mice were housed in SPF animal facilities with free access to water and food. The method used to isolate and cultivate BMDCs was adapted with slight modification from Inaba et al., 1992 [22]. Briefly, bone marrow cells were obtained from BALB/c mice by flushing tibias and femurs with RPMI 1640 complete medium (RPMI l640) (Gibco, Thermo Fisher Scientific Inc.) under aseptic conditions. After washing cells out of bone marrow, red blood cells were lysed by Tris ammonium chloride and bone marrow monocytes were cultured for 24 h in RPMI 1640 media containing 10% fetal bovine serum (FBS; Biological Engineering Materials Co., Ltd. Hangzhou Evergreen, China). After lysing and culturing, these cells were seeded into 24-well plates with 1.0 × 10 6 cells in each well in 1 mL of RPMI 1640/10% FBS supplemented with recombinant (rm) murine granulocyte macrophage colony-stimulating factor (GM-CSF) (20 ng/ml) and rm IL-4 (20 ng/ml), and incubated at 37°C with 5% CO 2 air. During a 6-day culture, nonadherent cells were discarded then rm GM-CSF and rm IL-4 were refreshed by replacing one-half of the supernatant volume with fresh medium every 2 days and collecting loosely attached cells as bone-marrow-derived dendritic cell for later use.

The flow cytometry assay for DCs surface markers
DCs cultured for 6 days were divided into three groups. The EPS was added to a final concentration of 0 (RPMI 1640) and 100 μg/mL for negative control and experimental groups, respectively. LPS was added to a final concentration of 1 μg/mL for the positive control. DCs were kept at 37°C for 24 h and then washed twice in cold PBS buffer (containing 2% (v/v) bovine serum albumin in PBS). After washing, the cells were placed in a round-bottom 96-well plate supplemented with 10% (v/v) goat serum and allowed to interact for 10 min. Subsequently, the cells were centrifuged and resuspended in PBS buffer at a concentration of 1 × 10 6 /mL. The sample was incubated for 30 min after addition of PE-CD11c, FITC-CD86 and FITC-MHC II to a 100 μL cell suspension. All steps were conducted on ice and in the dark. Cells were washed twice in PBS buffer and resuspended in 100 μL PBS buffer respectively for flow cytometric analysis on an EPICS XL flow cytometer with FACSDiva software (Version 6.1) for analyze data analysis (Beckman Coulter Inc., USA).

Determination of cytokine production
The loose adherent DCs were suspended with a sterile pipette and collected by centrifugation (300 g, 5 min), then were resuspended in RPMI 1640 with rm GM-CSF and rm IL-4. The cell concentration was adjusted to 1 × 10 5 /mL, and then the cell solution was transferred in a 24-well plate. EPS was added to each well with final concentrations of 0 (RPMI 1640, as negative control), 25, 50, 100, 150 and 200 μg/mL, and LPS was added to a final concentration of 1 μg/mL as positive control. The 24-well plate was incubated at 37°C and 5% CO 2 for 24 h. The supernatant was collected and frozen at -80°C until use. NO, IL-12p70, IL-10 and RANTES in supernatant were determined by the corresponding kits or mouse ELISA Sets.

Mixed lymphocyte reaction
After red blood cells were lysed by Tris-NH 4 Cl, BALB/c mouse spleen cells were resuspended and subsequently mixed with lymphocyte separation medium and RPMI l640, centrifuged at 300 g for 30 min. The lymphocyte was transferred to a 24-well plate and mixed with the BMDCs, which were treated with 0 (RPMI 1640, as negative control), 50, and 200 μg/mL EPS, and 1 μg/mL LPS (as positive control), respectively. After mixing, the cells were inactivated by mitomycin C, then incubated at 37°C for 72 h in 5% CO 2 air. By the last 4 h, 10 μL MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) was added to each well and the supernatant was discarded after centrifugation. Dimethyl sulfoxide (DMSO) was added to a 24-well plate and oscillated for 10 min. Optical density (OD) values were tested in the microplate reader at the 570 nm wavelength, and the reported results were the average of 5 technical replicates.

Statistical analysis
All data were analyzed using SAS statistical software (version 9.2) and they were expressed as mean ± SD. Significances among groups were tested using One-way ANOVA and the Wilcoxon rank-sum test. Differences were indicated statistically significant when P< 0.05.

Analysis of L. plantarum EPS
The homogeneity, Mw and composition of L. plantarum EPS lyophilized powder were analyzed (Fig 1). Its purity was about 97%. The SEC profile of the EPS displayed a single and symmetrical peak, indicating homogeneity of L. plantarum EPS. The EPS was composed of ribose, rhamnose, arabinose, xylose, mannose, glucose and galactose with a molar ratio of 2:1:1:10:4:205:215, respectively (Fig 1). The molecular weight was approximately 2.4×10 6 Da. There were little amounts of protein (28 μg/g) and endotoxins (0.20 EU/g) in the powder (S1 Table). The content of endotoxins were lower than that reported by Sheu et al [17], so any regulatory effect of EPS was not due to little amount of endotoxins.

Cytokines production in blood serum and intestinal fluid of BALB/c mice
Nitric oxide production. NO is one of the most important bioactive substances in the immune system, and is generated by immune system cells such as DCs [23]. As an intracellular messenger molecule it mediates a lot of biological functions and especially participates in activating macrophages [24]. The production of NO secreted in the serum and the intestinal fluid in treated mice by EPS or LPS is shown in Fig 2. There was an increase of NO production in the serum of mice for the 7-day period of L. plantarum EPS administration (P< 0.05) and no significant increase of NO production for all treatment of period of assessed in the fluid of mice that received L. plantarum EPS. It also showed NO production was lower in the EPS group compared to the LPS group (P< 0.05).
IL-12p70 and IL-10 production. IL-12p70 is a critical cytokine needed for the differentiation of the initial T cell differentiation into Th (T helper) 1 cells [25], while IL-10 maybe contribute to the differentiation of Th 2 cells [26]. After the mice were gavaged with L. plantarum EPS, the production of IL-12p70 and IL-10 in the serum and the intestinal fluid is shown in Fig  2. There was a significant increase of IL-12p70 production and a decrease of IL-10 production in the serum or the fluid of mice that received L. plantarum EPS for the 5-day and 7-day period of L. plantarum EPS administration (P< 0.05).
IL-12p70 production in the serum or the intestinal fluid of EPS group was higher than that in PBS group, while IL-10 production was lower than that in the PBS group. This indicated that EPS induced DCs in LP of intestine to produce more IL-12p70 and less IL-10. As an endotoxin LPS can cause a strong immune response or damage. Compared to LPS group, lower changes of IL-12p70 and IL-10 production in EPS group did not cause a strong immune response or damage.
RANTES production. RANTES is secreted by ectopic endometrial cells, such as DCs, and activated T cells, causing the aggregation of more macrophages and T cells, and thus the secretion of more cytokines and chemokines [27,28]. Concentrations of RANTES in the serum and the intestinal fluid after the mice were gavaged are shown in Fig 2. RANTES production increased significantly compared to PBS group in the fluid of mice that received L. plantarum  EPS for the 5-day and 7-day period of L. plantarum EPS administration but lower than that in LPS group (P< 0.05) while there were no changes in the serum at all-time points of EPS administration. It was indicated that the intestine was the main interact site for EPS cross talk with DCs.
In this study, L. plantarum EPS had a great impact on the production of NO, IL-12p70, IL-10 and RANTES related to DCs in the serum and the intestinal fluid of BALB/c mice, which may result in activation of macrophages and inducing differentiation of Th0 cells to Thl cells, especially enhancing intestinal mucosal immunity. LPS is a common endotoxin that produces a strong immune response or damage in the body and is often designed as positive control in studies [17,18]. Our results showed that L. plantarum EPS had less impact on chemokine production than LPS, indicating that the EPS modulated a moderate immune response. Subsequently, we performed the in vitro experiment about the maturation of DCs treated by the EPS in order to understand the change of these cytokines production in the serum and the intestinal fluid. MHC II and costimulatory molecule CD86, both necessary in signal transduction for DC to recognize and uptake antigens, are the best indicators of DC immune function. The results are shown in Fig 3. MHC II expression of DCs treated with RPMI 1640 was 60.5%, while the group treated with 100 μg/mL EPS and LPS were 68.4% and 69.7%, respectively. Results showed that EPS could increase DCs surface MHC II molecule expression, and there was lower expression in the EPS group than the expression in the LPS group.

Phenotype and cytokine production of mouse BMDCs
CD86 expression was 66.7% in the RPMI 1640 group, 72.1% in the 100 μg/mL EPS group, and 72.9% in the LPS group (Fig 4). The results showed that EPS could significantly improve the expression of DCs costimulatory molecule CD86 (P< 0.05), not as much as that in LPS group.
As immune cells, DCs are the most powerful antigen presenting cells (APCs), and their function is related to their maturity [29]. Mature DCs can express a great amount of surface costimulatory molecules such as CD40, CD80, CD86 and MHC II, while immature DCs express little of these molecules. According to these surface markers, DCs can be divided into different types, with mature and immature being two of the major groups [30]. MHC II molecules are very important to the classification of DCs [31]. EPS was proved to promote the expression of MHC II molecules of DCs, suggesting that it may also induce the maturation of DCs. CD80, CD86 and CD40 are signs of mature DCs, and play indispensable roles in stimulating the immune response. CD86 activates DCs and Th0 cells, while memory T cells provide costimulatory signals. T cell activation requires two stimuli: (1) the binding of the T cell receptor (TCR) with antigen peptide-MHC molecule complexes, and (2) the combination of APCs surface with co-stimulatory molecules located on TCRs (CD40 and CD40L, or the interaction between either CD80 and CD28 or CD86 and CD28). The T cell response is said to be in the anergic state if the T cell recognition is processed by antigen-containing APC cells without allowing for the co-stimulatory molecules to promote the auxiliary signal [32]. Our experimental results showed that EPS induced DCs to produce more MHC II and CD86, which probably involved in enhancing DCs antigen presentation ability and in turn promoting the proliferation of T cells.
Nitric oxide production. The quantity of NO secreted by EPS and LPS-treated DCs is shown in Fig 5(A). NO secretions from DCs treated for 24 h with EPS and LPS were all increased compared with NO secretions from the RPMI 1640 group. The result showed that NO secretion was 298.38 ± 1.13, 308.03 ± 0.28, 315.63 ± 0.20, 329.52 ± 0.45, and 340.03 ± 2.32 nmol/mL for the 0 (RPMI 1640), 50, 100, and 200 μg/mL EPS and LPS-treated DCs groups, respectively. EPS stimulated DCs to secrete NO, but secretions were lower in the EPS-treated group than the LPS-treated group.
Th0 cells play an important role in the immune response by secreting different ratios of Th1-and Th2-type cytokines. Th1-type cytokines such as IFN-γ and IL-2 play a role in phagocytosis by activating cytotoxic properties of effector cells. Th2-type cytokines such as IL-4 and IL-10 can stimulate B cells to produce antibodies and DCs also can interact with B cells to promote Th2 response [33]. DCs regulate differentiation of Th0 cells into Th1/Th2 cells through secretion of cytokines by DCs and T cells in order to maintain a balance between the two types of cells [34]. Thl and Th2 cell-mediated responses not only play different roles in the body's defense mechanism, but are also involved in differing immune pathology. Continually strong Thl cell immune response may lead to some organ-specific autoimmune diseases such as experimental allergic encephalomyelitis (experimental autoimmune encephalomyelitis, EAE) [35], Type I diabetes [36] and so on. Injecting the body with a special peptide specific to diabetes and DCs cultivated in vitro can effectively prevent the onset of diabetes. This is because when the peptide meets with the DCs, it can induce the expansion of Th2 cells, which are required for the peptide to induce a shift in the Th1 to Th2 immune response. The polysaccharide isolated from the cell walls of seaweeds has been proven to have potential clinical applications in the treatment of autoimmune diseases by keeping immune homeostasis [37]. In the treatment of autoimmune diseases, DCs are recognized as a great value immune cells [38]. DCs from different sources can cause different types of T cell responses, which can be divided into myeloid (DC1) and lymphoid (DC2) categories according to sources, phenotype and cytokine. DC1 induces Th0 cells to Thl-type cells, while DC2 induces Th0 cells to Th2-type cells [39]. However, some research found that myeloid DCs tended to induce Th2-type response, and lymphoid DCs tended to Th1-type response [25]. In a mouse transplantation model, liver DCs induced Th2-type response, and bone marrow DCs induced Th1-type cytokines such as IFN-γ production [40]; Compared with spleen DCs, human intestinal mucosal Peyer's lymph node DCs strongly stimulate T cell activation and produce Th2-type cytokines, but Thl-type cytokines are only weakly induced [41]. Regulation of different types of DCs on Th1 and Th2 response were sometimes different probably because of species differences between humans and mice, or methods of DC purification and culture in vitro. However, some researchers thought that this was because of differences in DC IL-12 secretion in various developmental stages [42]. IL-12 was a key member of the third DC signal in regulating Th1/Th2 response, and could determine the direction of Th0 cell differentiation [43]. IL-12 is critical to the differentiation of Th0 cells to Thl cells, while IL-4 tends to promote Th2-type cell response in general. Other cytokines in the surrounding environment also contribute to the development of DCs, for example, IFN-γ and IL-2 can induce immature DC to differentiate into DC1, which not only express costimulatory molecule CD86, but also secrete large amounts of IL-12, and promote the differentiation of Th0 cells to Thl cells. Th2 cytokines, such as PGE2, promote differentiation of immature DC to DC2. PGE2 can express a great amount of costimulatory molecules but little of IL-12, and can induce differentiation of Th0 cells to Th2 cells. IL-10, unlike PGE2, can inhibit the development of immature DCs in the early stage and keep DCs expressing costimulatory molecules and secreting IL-12 at a low level.
In this experiment, IL-12 secreted by EPS-treated DCs in supernatant was higher than that in the RPMI 1640 group. On the contrary, IL-10 secreted by EPS-treated DCs was lower than that in the RPMI 1640 group. There's a strong possibility that the EPS can induce differentiation of Th0 cells to Thl cells, and be able to induce the secretion of RANTES-Thl-type. Thus, the EPS has potential for inducing immune responses of Th0 cells to Thl-type cells and promoting innate immunity through the transition to adaptive immunity by regulating the types of cytokines and chemokines secreted by DCs.
RANTES production. Concentration of RANTES is shown in Fig 5(D). For treatment with 0 (RPMI 1640), 25, 50, 100, 150 and 200 μg/mL EPS, RANTES secretion was 60.91 ± 9.48 pg/mL, 64.46 ± 7.53 pg/mL, 68.80 ± 6.29 pg/mL, 70.62 ± 6.62 pg/mL, 74.99 ± 2.77 pg/mL and 81.05 ± 4.90 pg/mL, respectively. RANTES content in the 200 μg/mL EPS group was higher than that in the RPMI 1640 group (P < 0.05), but not as high as that in LPS group. Thus, 200 μg/mL EPS could promote DCs to secrete RANTES and could induce Thl-type based immune response and the transition of innate immunity to acquired immunity. RANTES plays a key role in migration and activation of leukocyte, resulting in the recruitment of CD4 + and CD8 + T cells to sites of inflammation [44], so these results suggested that EPS had the capability of inducing the migration and activation of Th1 cell by secreting RANTES of DCs treated with EPS.
Allogeneic lymphocyte proliferation. EPS induced by stimulation of BMDCs allogeneic lymphocyte proliferation to reflect the impact of antigen-presenting ability of DCs. EPS concentration of 0 (RPMI 1640), 50, 200 μg/mL, and LPS induced lymphocyte proliferation rates were 99.99 ± 16.37%, 118.85 ± 24.13%, 239.34 ± 24.63% and 371.31 ± 38.65%, respectively ( Fig  6). There was no significant increase in lymphocyte proliferation when treated with 50 μg/mL of EPS (P> 0.05) as compared with the RPMI 1640 group. However, when treated with the 200 μg/mL concentration of EPS, lymphocyte proliferation was significantly increased (P< 0.05), although it was still lower than that in the group treated with LPS (P< 0.01).
In this experiment T cells stimulated by DCs treated with EPS or LPS proliferated higher than the control group. This is because DCs in the RPMI 1640 group expressed a moderate level of MHC II molecules and low levels of CD86 and CD40 costimulatory molecules. As a result, T cell proliferation cannot be activated because of the lack of the necessary second signal. DCs treated with EPS or LPS became mature and provided T cells with the necessary surface molecules, resulting in proliferation of T cells.
At present, there are three signals involved in DC activating T cells to produce specific immune response: the first signal is the specific combination between the DC surface, MHC molecule/peptide complex and the Th cell surface TCR/CD3; the second signal is the interaction of coordinated stimulation molecules between the surface of DC and T cells [45]. These two signals together promote T cell activation and initiate the body's immune response. In this experiment, T cell proliferation was more highly stimulated by DCs treated with low or high concentration EPS than by DCs in the RPMI1640 group which were not treated with EPS. Without the treatment of EPS the DCs expressed MHC II and CD86 at a low level, so T cell proliferation cannot be activated. T cell proliferation was improved with EPS treatment due to maturation of the DCs resulting in adequate surface molecule expression. Kaliński proposed a third signal: at the start of immune response, DCs could express surface molecules selectively which could induce T cell polarization. Some of these molecules combined with the cell membrane, and some were soluble [42]. At the present time, IL-12 has been most studied, and it may balance the number of Thl and Th2, including regulatory T cells generation.

Conclusions
Our study demonstrates that L. plantarum EPS has an immune modulation on DCs for the first time. We have shown that the EPS was homogeneous with Mw of 2.4×10 6 Da. It increased NO production in the serum, enhanced IL-12p70 production in the serum as well as in the intestinal fluid, promoted RANTES production in the intestinal fluid and decreased IL-10 production both in the serum and in the intestinal fluid. In vitro L. plantarum EPS increased the production of NO, IL-12p70 and RANTES while reduced the secretion of IL-10. Furthermore, the EPS also up-regulated the expression of MHC II and CD86. Finally, DCs treated by the EPS promoted proliferation of T cells. L. plantarum EPS can promote maturation of DCs in BALB/ c mice. We will further study the structural features of the polysaccharide and its molecular mechanism on modulation of immune function.